1. Field of Use
This invention relates to direct current brake means for alternating current variable speed reversible wound rotor motors, such as are used on hoists, cranes, rolling mills or the like.
2. Description of the Prior Art
In certain industrial equipment, such as cranes, hoists, steel rolling mills and the like, it is advantageous to use variable speed, reversible electric motors capable of handling heavy, and sometimes overhauling, loads.
Such motors when applied to hoists, for example, are provided with control means, including controllable SCRs, which enable motor operation in the powering mode (both for hoisting and lowering), in the regenerative mode (during lowering only), and in the braking mode (during both hoisting and lowering). In the powering mode (wherein speed and torque are in the same direction), direction of motor rotation is determined by control of the phase sequence of the alternating current voltage applied to the motor stator (primary) windings by the SCRs and slip speed (or torque) is controlled by regulating the magnitude of the applied stator voltage.
In the regenerative mode (wherein speed and torque are in opposite directions and the motor functions as an induction generator with negative slip), the direction of motor rotation is determined by the load, and torque and speed are determined by inherent motor characteristics.
In the braking mode, it is known that braking can be accomplished by conventional counter-torque techniques or by applying direct current from an outside source to the stator (primary) winding of the wound rotor motor and the prior art contains several patents which disclose completely separate and independent direct current sources used for this purpose, such as, U.S. Pat. Nos. 3,166,701; 3,302,086; 3,281,633; and 3,313,992, for example.